1. Field of the Invention
The present invention relates to a gas injecting device of a cylinder for an injection molding machine, and a method for controlling an amount of gas injected into a barrel of the injection molding machine.
2. Description of the Related Art
Generally, with an injection molding machine for injection (extrusion) molding a material into an injection (extrusion) molded article, microcellular foam processing has been used for weight reduction of the article while enhancing reproducibility, and dimensional precision of the article. For this purpose, conventional microcellular foam processing uses a technique for producing micro bubbles in the polymeric material for injection molding in order to reduce the weight of the plastic article. One of the conventional injection molding machines using this technique, as disclosed in PCT/US1999/026192, includes a gas injecting device equipped at one side of a cylinder of the injection molding machine, and having a small-sized gas spraying orifice formed at a distal end of the gas injecting device to finely inject gas in order to provide a mixture of the gas and a molten resin.
However, the conventional injection molding machine of the disclosure has problems in that, when an internal pressure of the cylinder is lower than an injection pressure of the gas during injection of the gas into the cylinder, the molten resin flows backwards through the gas injecting device, and often solidifies, causing clogging of the gas spraying orifice.
In other words, for the conventional microcellular foam processing by the injection molding machine of the disclosure, since the gas is injected from the gas injecting device in a state of being opened, the high internal pressure of the cylinder can cause backflow of the molten resin through the gas injecting device, and in this case, the molten resin is solidified to cause the clogging of the gas spraying orifice, thereby failing to ensure accurate injection of the gas, and providing defective products. In addition, since the solidified resin is difficult to remove from the conventional foam processing, cleaning of the solidified resin is laborious, and time consuming, thereby lowering work efficiency.
Briefly, conventional microcellular foam processing has problems in that the injection pressure of the gas is lowered due to the orifice phenomenon, causing the backward flow of the molten resin through the gas injecting device, and in that, when a screw positioned within the cylinder is rotated, the internal pressure of the cylinder is increased above the injection pressure of the gas, causing the backflow of the molten resin towards the gas spraying orifice of a low pressure to cause the clogging of the gas spraying orifice. In addition, with the conventional microcellular foam processing, the gas is directly injected at a high pressure through the gas spraying orifice, so that the gas cannot be effectively mixed with the molten resin, thereby failing to obtain an article of an accurate weight.
Conventional injection molding is an injection process wherein, after melting a resin, in particular, a polymeric material, at a high temperature, the molten resin is injected into a cavity of an injection mould at a high speed with a considerably high pressure. It is a fundamental principle of the injection molding that an injection molding machine is attached to the mould, and is operated to inject the molding material (polymeric material) such that the mould is filled with the material.
When injection molding the polymeric material into the article which is usually formed with a thick portion due to requirement for reinforcement of the article, a molding process is employed, in which air or gas is injected into the molten resin during the injection molding in order to prevent the article from being defective via deformation or shrinkage of the article due to retardation of cooling at the thick portion.
In other words, in the conventional injection molding machine, the gas injecting device is equipped at one side of a barrel to inject gas, and has a small-sized gas spraying orifice formed at a distal end of the gas injecting device to finely inject gas into the barrel through the gas injecting device, so that, when the gas and the molten resin are injected into the barrel, bubbles are formed in the molten resin as the gas and the molten resin are mixed in the barrel.
Typically, it has been considered that, when the article is injection molded by the injection molding machine, it can have suitable properties through control of injection time and amount of the gas injected along with the resin into the barrel according to characteristics of the article or kinds of the resin used as a raw material of the article. However, in the conventional injection molding, since the amount of the gas injected together with the resin into the barrel is not controlled irrespective of the kinds of the resin or the characteristics of the article, the injection molding is not performed corresponding to the kinds of the resin or the characteristics of the article.